Pedigree dogs are suffering from genetic diseases following years of inbreeding, an investigation has found.
A BBC documentary says they are suffering acute problems because looks are emphasised over health when breeding dogs for shows.
The programme shows spaniels with brains too big for their skulls and boxers suffering from epilepsy.
The Kennel Club says it works tirelessly to improve the health of pedigree dogs.
Pedigree animals make up 75% of the seven million dogs in the UK and cost their owners over £10m in vets' fees each week.
The programme, Pedigree Dogs Exposed, says dogs suffering from genetic illness are not prevented from competing in dog shows and have gone on to win "best in breed", despite their poor health.
It says physical traits required by the Kennel Club's breed standards, such as short faces, wrinkling, screw-tails and dwarfism, have inherent health problems.
Other problems occur because of exaggerations bred into dogs by breeders trying to win rosettes, it adds.
The programme shows a prize-winning cavalier King Charles spaniel suffering from syringomyelia, a condition which occurs when a dog's skull is too small for its brain.
It also features boxers suffering from epilepsy, pugs with breathing problems and bulldogs who are unable to mate or give birth unassisted.
It says deliberate mating of dogs which are close relatives is common practice and the Kennel Club registers dogs bred from mother-to-son and brother-to-sister matings.
Scientists at Imperial College, London, recently found that pugs in the UK are so inbred that although there are 10,000 of them, it is the equivalent of just 50 distinct individuals.
Steve Jones, professor of genetics at University College London, said: "People are carrying out breeding which would be first of all entirely illegal in humans and secondly is absolutely insane from the point of view of the health of the animals.
"In some breeds they are paying a terrible price in genetic disease."
RSPCA chief vet Mark Evans was interviewed for the programme.
He said: "The welfare and quality of life of many pedigree dogs is seriously compromised by established breeding practices for appearance, driven primarily by the rules and requirements of competitive dog showing and pedigree dog registration."
But Kennel Club spokeswoman Caroline Kisko said it is "working tirelessly" to help improve the health of pedigree dogs.
"Any dog may be shown but it is up to the judge to decide if it fits the breed standard," she said.
"It is when characteristics become exaggerated that health problems can occur.
"This is something that the Kennel Club does not encourage and actively educates people, including judges, against doing as part of its Fit For Function, Fit For Life campaign."
Pedigree Dogs Exposed will be shown on BBC One at 2100 BST on Tuesday 19 August